Does ammo have an Expiration date? How old is too old?
It expires as soon as you fire it!
But seriously, I'm good keeping practice ammo for decades in moderate temp and humidity as long as it's kept it in sealed containers.
For self/home defense rounds, they are probably in the gun or at least in mags or a nearby drawer. Since it's exposed to more variations in the environment, it's good to rotate that ammo with fresh rounds annually.
Take the old to the range as an ongoing verification it still cycles in your gun, then replace it with new factory ammo.
That's how I do things. I'm sure others might have differing opinions/practices.
Too old to me is when it's corroded or in the case of .22, the projectiles are loose in the cartridge.